Liquid crystal display

ABSTRACT

A liquid crystal display having a liquid crystal display panel having a plurality of liquid crystal cells, and a back light that irradiates the liquid crystal display panel with irradiation light. The back light includes a light source, and a light guide plate having one side arranged adjacent the light source and having a planar surface with a plurality of small concaves. The planar surface is almost parallel to a liquid crystal cell face, and a plane shape of the small concaves is almost rectangular.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This is a continuation of U.S. application Ser. No. 09/257,062,filed Feb. 25, 1999, which is a continuation-in-part of copendingapplication Ser. No. 08/791,513, filed Jan. 30, 1997, by some of theinventors herein and having a common assignee, now U.S. Pat. No.5,961,198, issued Oct. 5, 1999, the subject matter of which isincorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a liquid crystal display using acomponent for liquid crystal displays, such as rear illuminationequipment.

[0003] In recent years, the implementation of personal computers,inclusive of so-called word processors, in a small size has beenpromoted, and portable type personal computers, known as lap-top type ornotebook type computers, are now widely used. In such a portable typepersonal computer, a liquid crystal device is commonly used as a displayunit. In this regard, there is an increasing tendency for adopting acolor display in portable type personal computers. In line with such atrend, a backlighting type display device is coming into wide use, inwhich a light source is disposed at a rear side of a liquid crystaldisplay screen for lighting the whole display screen from the rear orback side. Needless to say, the backlighting light source for the colorliquid crystal display device is required to emit light with highluminance. Besides, it is necessary to illuminate the display screenwith uniform luminance over the whole planar surface thereof. Luminanceof the backlighting can easily be increased by increasing the luminanceof the light source. However, taking into consideration the fact that aportable-type personal computer or word processor or the like is usuallyoperated br using a battery or storage cell, a limitation concerning thevoltage supply is necessarily imposed on any attempt to increase theluminance of the light source. Stated in another way, no other effectivemethod or measures for increasing the luminance of the liquid crystaldisplay screen have been proposed to date.

[0004] For having better understanding of the present invention, adescription will first be made in some detail of conventional liquidcrystal display devices, such as disclosed, for example, inJP-A-4-162002 and JP-A-6-67004. FIG. 2 shows a lateral source typebacklighting device employed conventionally in a liquid crystal displaydevice known heretofore. Referring to FIG. 2, a lamp, such as acold-cathode discharge tube or a hot-cathode discharge tube, is employedas a light source 1 which is disposed at and along one lateral side of alight guide plate (also known as optical waveguide plate) 2 which ismade of a light-transmissive material. Here, an optical scattering layer3 from which light is scattered and reflection sheet 4 that causes lightto reflect are provided on the underneath side of said light guide plate2. And, a diffusion sheet 5, that consists of synthetic resins ofmilk-white color that have an optical scattering effect, is provided inthe area over the surface of said light guide plate 2 to pass and toilluminate the whole face with a uniform brightness. In addition, afirst condensation sheet 6 to converge diffused light to some extent onthe face and to improve the brightness of the front face of the displayand a second condensation sheet 7 are arranged above the diffusion sheet5. As to the optical scattering layer 3, which is shown in more detailin FIG. 3, it consists of a plurality of ink dots 8, formed of opticalscattering materials, such as oxide titanium, arranged on the surface oflight guide plate 2. As the distance increases from the light source 1,the optical intensity from light source 1 is reduced. Therefore, as thedistance increases from the light source 1, as shown in FIG. 3, the areaof the ink dots 8 is increased.

[0005] As described above, there is a problem of the brightnessdeclining due to the loss of optical scattering in conventionalillumination equipment. The reason is because light is emitted fromlight source 1, conducted to light guide plate 2, scattered by opticalscattering material 8 that is contained in the optical scattering layer,passes through a diffusion board later, and then irradiates a liquidcrystal element.

[0006] There is a light guide plate not using ink dots, to solve thisproblem, as described in JP-A-9-269489. This light guide plate has asmall convex or a small concave area formed on the surface thereof.These small convex or small concave areas reflect light, and a liquidcrystal element is illuminated thereby. But the shape and distributionof these areas were not optimized, and so there was still room for afurther improvement in brightness.

[0007] In addition, as one of the narrow advances that led to animprovement in the brightness of a liquid crystal display, it has beenproposed that the permeative rate of a polarization filter should beraised. The polarization filter is an element that is arranged between aliquid crystal cell and the back light and which has the function ofpassing only a specified polarization light into a liquid cell. Thepolarization filter can be manufactured by adsorbing a dichroic materialin the micell pipe of a macromolecule film that generally arranges amicell in a constant direction. As a macromolecule film, polyvinylalcohol is used. Between rollers on which this polyvinyl alcohol spinsat a different speed, it is drawn about 3-5 times in the constantdirection.

[0008] The micell of a drawn PVA (polyvinyl alcohol) is arranged in thedrawing direction, and the arranged film has a strong double refraction.There are halogen materials, such as a iodine and a dyestuff, as amaterial to give dichroism. By adsorbing the above material in the filmbeing drawn, polarization characteristics are expressed. As for theabove polarization filter, a polarization separation function is gainedeasily. But the permeative rate is small, being 50% or less. Because adichroic material is used theoretically, polarization light that isorthogonal with the polarization light that is transmitted is absorbed.Therefore, it is a present condition that 50% or more of the opticalenergy is lost by a polarization filter, and the brightness of a liquidcrystal display element using such a filter is remarkably reduced as aresult.

[0009] It has been proposed to adopt a method of using a polarizabilityfilm of a reflection type as a means of obtaining improved brightness. Apolarizability film of a reflection type is a film that has a propertysuch that all polarization components other than a polarizationcomponent of a specific kind are reflected and only the specificpolarization component is passed, like a cholesteric liquid crystalfilm, etc. That is, the polarizing natural light that comes out througha light guide plate is applied to a polarizability film of a reflectiontype, whereby only a specific polarization component is transmitted, andall other polarization components are reflected. A reflectedpolarization component is reflected again by a reflection board later,the polarization state is changed, it is applied to the polarizabilityfilm of a reflection type again, and only the specific polarizationcomponent is allowed to pass through. By repeated reflections, allcomponents of light can be used.

[0010] A method of using a cholesteric liquid crystal film representinga polarizability film of a reflection type has been proposed inJP-A-3-45906 and the JP-A-6-281814. The cholesteric liquid crystal filmconsists of optical active layers of a polymer material that hascholesteric regularity.

[0011] The cholesteric liquid crystal film transmits only a circularpolarization component of the same direction as the spiral direction ofa cholesteric layer in the polarizing natural light that comes outthrough a light guide plate from the light source, and the circularpolarization component of a reverse direction is reflected. Therefore,when has the structure shown in FIG. 16, a reflected circularpolarization component is reflected again with a reflection sheet, it isreturned, it returns it to a state that is close to natural light, andthe cholesteric liquid crystal film is entered again. By a repeat ofthis cycle, all components of light can be used. When a ¼ phase sheet isformed on the surface at which light appears on this cholesteric liquidcrystal film, a circular polarization component is converted into astraight line polarization, and so the arrangement can be used as rearluminescence equipment for a liquid crystal display.

[0012] The brightness of a liquid crystal display using a cholestericliquid crystal film becomes double the brightness of the liquid crystaldisplay which uses an ordinary polarization filter from theoreticalviewpoint. But, in case a light guide plate of the ink dot type, that iswidely used at present, and a cholesteric liquid crystal film arecombined, the results are unsatisfactory. The reason is because itincurs a loss due to the scattering mentioned above, and when the lightwhich does not pass through the cholesteric liquid crystal film isreflected, the circular polarization that is injected into the lightguide plate again is reflected (diffusion) by an ink dot, and loss byscattering occurs, and the polarization state also is degraded further.And, as for the dot, the size is quite large, and so it is necessary touse a diffusion sheet in combination to prevent dot visibility, with theresult that the brightness improvement effect declines further. In caseit is combined with a prism sheet that optimizes the angle ofdistribution of the light that comes out and that improves front facebrightness, the brightness improvement effect is not more than 10%.

[0013] However, a method of combining the light guide plate that isformed with a grating of grooves in the surface thereof and acholesteric liquid crystal film has been proposed in JP-A-9-102209. Thiscombination produces high brightness, because it has a small loss byscattering, and comparing the brightness improvement effect with that ofa printing dot, it is high.

[0014] But as for the light guide plate that is formed with gratinggrooves, it is difficult to control the brightness distribution in the Xdirection, and it is expected that problems with the cost of metal moldmanufacture and with meeting an appointed date of delivery is alsolarge. In addition, there is a fear of a low mass production. The reasonis because, from the point of view of surface roughness, soft metals,such as brass, must be used as a metal mold material. In addition, it isexpected that moire will easily occur between periodically formedgrating grooves and a liquid crystal cell. And, as for the gratinggrooves, the period is quite large, it is necessary to use a diffusionboard in combination, and so the brightness improvement effect has atendency to decline further. In case it is combined with a prism sheetthat optimizes the angular distribution of the light that comes out andthat improves the front face brightness, its brightness improvementeffect is not more than 20%.

[0015] To irradiate the liquid crystal element side with light from alight source using scattering provided by an ink dot with conventionalequipment such as that described above, light is absorbed at the time ofbeing scattered by the ink dot, and so there is a limit to theimprovement in efficiency available with such a system. And, with regardto a light guide plate not using ink dots, the shape and distribution ofthe light are not optimized, and so there is room for an improvement inbrightness. In the combination of a light guide plate using the ink dotmethod and a cholesteric liquid crystal film, the brightness improvementeffect is as low as 10%. While the combination of the light guide platehaving grating grooves and a cholesteric liquid crystal film, thebrightness improvement effect is only as high as 20%, and so it isdifficult to control the brightness distribution and to economicallymanufacture the light guide plate.

SUMMARY OF THE INVENTION

[0016] To solve the problems inherent in conventional devices, thepresent invention is proposed.

[0017] It is an object of the present invention to provide a liquidcrystal display that can improve brightness without enhancingconventional faults and without increasing the brightness of the lightsource.

[0018] In this regard, by using a light guide plate that has formedtherein several small concaves (called dots in the followingdescription) to convert the direction of light waves in the light guideplate from a specified direction toward the liquid crystal display, andby properly designing the plane shape and a section inclination angle ofthe dots, the objects of the present invention can be achieved.

[0019] A reflection sheet is arranged underneath the light guide plate.In addition, illumination light that has a proper angular distributionfrom the light emitting face to the prism sheet that has a proper prismvertical angle according to a requirement can be used to irradiate adisplay element.

[0020] In addition, the dots are arranged at random to prevent moirefrom occurring. And, light comes out of a dot with a specified densitydistribution, and so equalization of the brightness distribution oflight is achieved.

[0021] In addition, between the light emitting surface and liquidcrystal unit, a polarizability film of a reflection type (cholestericliquid crystal film, ¼ phase sheet, etc) is arranged, with a result thatbrightness is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1(a) is the perspective view of the illumination equipmentthat is used for the liquid crystal display according to an embodimentof the present invention and

[0023]FIG. 1(b) is an enlarged view of a portion thereof.

[0024]FIG. 2 is a sectional view of a conventional light guide plateusing a dot printing method.

[0025]FIG. 3 is a diagram of a conventional dot printing arrangement.

[0026]FIG. 4 is a diagram of the cutting direction of a light guideplate.

[0027]FIG. 5 is a diagram used to explain a light locus in theillumination equipment of the light guide plate that is used for thepresent invention.

[0028]FIG. 6 is a diagram of the dot shape of a light guide plate thatis used for the present invention.

[0029]FIG. 7 is a diagram of the section inclination angle of a lightguide plate that is used for the present invention.

[0030]FIG. 8 is a graph of the surface coarseness RA and front facebrightness of a light guide plate that is used for the presentinvention.

[0031]FIG. 9 is a graph forming a dot reflection validness area ratiodistribution diagram of a light guide plate that is used for the presentinvention.

[0032]FIG. 10 is a diagram of a dot random arrangement of a light guideplate that is used for the present invention.

[0033]FIG. 11 is a diagram of the section inclination angle of a lightguide plate that is used for the present invention.

[0034]FIG. 12 is a graph of the section inclination angle and front facebrightness of a light guide plate that is used for the presentinvention.

[0035]FIG. 13 is a graph forming a dot reflection validness area ratiodistribution diagram of a light guide plate that is used for the presentinvention.

[0036]FIG. 14 is the flow chart that shows a method of manufacturing alight guide plate that is used for the present invention.

[0037]FIG. 15 is a perspective view of the liquid crystal displayaccording to an embodiment of the present invention.

[0038]FIG. 16 is a diagram of a liquid crystal display using acholesteric liquid crystal film.

[0039]FIG. 17 is a side sectional view of a liquid crystal display usinga cholesteric liquid crystal film.

[0040] FIGS. 18(a) and 18(b) are graphs showing the effect of acholesteric liquid crystal film of example 2 of the present invention.

[0041] FIGS. 19(a) and 19(b) are graphs showing the effect of acholesteric liquid crystal film of comparison example 1.

[0042] FIGS. 20(a) and 20(b) are graphs showing the effect of acholesteric liquid crystal film of comparison example 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] The perspective view of an embodiment of the present invention isshown in FIG. 1(a).

[0044] To convert the direction of a light wave in the light guide plateto a specified direction, dots 10, that are made up of several smallconcave areas, are formed in the under surface 9 of the light guideplate of the light source equipment that is used for the presentinvention. Each dot is a small concave area that consists of a dotbottom that exists in the surface 11 (face from which it comes out) andincludes a dot bottom part 12 (extending at an angle relative to face 13of 5° or less) spaced from the underneath surface of the light guideplate and dot side sloping surfaces 14, as shown in FIG. 1. (However,because the view in FIG. 1(a) is from the inside of a light guide plate,each small concave appears in the drawing as a small convex, as shown inthe enlarged view in FIG. 1(b)).

[0045]FIG. 4 shows the various directions relative to the light guideplate 2 to aid in the following explanations. In a case in which thelight source 1 is a linear light source, a plane that is perpendicularto the light source is designated as a light guide plate section 15. Thedirection that is parallel to the axis of the light source is designatedas the X direction 16, and the direction that is parallel to the face 13and is perpendicular to the light source, representing the directionfrom which light comes out from the light guide plate, is designated theY direction 17.

[0046]FIG. 5 shows the locus of the light guide plate for a light wavethat progresses inside of the light guide plate 2 of the presentinvention. The light from light source 1 is projected into the lightguide plate 2 as incident light 19 at one end 18 of the light guideplate 2, and it becomes light waves 20. The light waves 20 progresstoward the other end 21, repeating all reflections between theunderneath surface of the light guide plate and the top face 13 at whichlight is emitted from the light guide plate. Light 23 that is directedat a dot inclination face 22 includes light waves 20 that reflect offthe inclined surface and are directed to the face 13, are refracted atthe surface 13 of the light guide plate, and emerge as light 24 fromface 13. On the other hand, the light that is not reflected at thesurfaces 22 becomes dot slope transmission light 25, which is reflectedat the reflection sheet 4, and is injected into light guide plate 2again. Part of this light which is reflected by reflection sheet 4 comesout of the face 13, and the remainder becomes light guide plate light 20again. Therefore, it starts in light guide plate 2 gradually as lightwaves 20, and it is reflected upwardly through the light emittingsurfaces 13 by the dots so that the liquid crystal display elements canbe illuminated by properly arranging the dots.

[0047]FIG. 6 shows the shape of a suitable dot as seen from above in aprojection of the dot on face 13 of the light guide plate. The dot issubstantially rectangular in form with slanting side surfaces androunded corners 26. The reason for this shape is because the area of thedot reflection face 22 can be increased relative to a circular shape ora square shape having the same dot base area, and so the number of dotscan be reduced. That is, the degree of freedom of dot distribution alsois improved, and creation of mask and dot coordinate data also becomeseasy. And, the roundness 26 has an effect of reducing scattering andraising the front face brightness. As shown in FIG. 6, the length of along side is referred to as a dot bottom unit dot length 28, and thelength of a short side is referred to as a dot width 29.

[0048] The direction of a dot is arranged so that the slope 30 of a dotlength side 28 is substantially in parallel with the X direction 16. Thereason for such an arrangement is because the light from light source 1can be reflected efficiently, because the area 30 is wider than theslope 31 of the short dot side.

[0049] As for the size of a dot, it is desirable that the dot length 28is 80-800 μm. The reason for this is that, when the length 28 becomeslarger than 800 μm, and users of a personal computer, for example, lookat a liquid crystal display, the shape of a dot formed out of acharacter and a pattern appears at the light guide plate, and sodistinguishing between a dot, a character and a figure becomesdifficult. When the length 28 becomes smaller than 80 μm, the dotsappear in large numbers, and the mask and dot coordinate data creationform shapes that make it difficult to distinguish. When the cost of maskmanufacture is considered, the optimum value is 100 μm or more, and whendisplay appearance is considered, 300 μm or less is desirable.

[0050] As for the dot width 29, it is desirable that it is 60 μm orless. The reason is because the rate of dot slope 14 that is occupied inthe dot area when the length 29 becomes larger than 60 μm is reduced,and it becomes difficult to obtain a sufficient reflection face. Inaddition, it is desirable that, to gain a brightness that is alsosufficient in the four corners of the light guide plate 2, the width 29should be 45 μm. And, when the width 29 is 20 μm or more, an inexpensivemask can be used. The exposure machine, while at the lower limit, sinceit uses a mask during exposure, has a problem to achieve the requiredprecision of the resist layer. In addition, 30 μm or more is desirable.

[0051] As for the depth of a dot, 5-20 μm is proper. The reason isbecause the dot shape is hard to form. That is, to obtain a depth of 20μm or more, a viscous resist material must be used, and the plane of thesurface of a dot formation face declines, and when forming light guideplate 2, the transcription declines. When the depth of a one-sided dotis as small as 5 μm, the rate that is occupied in the dot area of dotslope unit 14 reduces, and the required brightness cannot be achieved.And, as for the above thickness when it is considered that a liquid-formresist is used, 6-12 μm is optimum.

[0052] As for the section inclination angle of the present invention,20°-35° is suitable, and an optimum value is 28°±4°. The sectioninclination angle will be explained by reference to FIG. 7. FIG. 7 is across section through the 25 center of a dot, and a dot section formedby a dot cutting plane that is parallel to the light guide plate cuttingplane 15 of FIG. 4 is shown. Determination of the cutting plane likethis is done since the slope 30 of the dot length side causes a changein the direction of light.

[0053] A section inclination angle is calculated by the followingmethods. As shown in FIG. 7, the dot depth is divided equally into threedivisions, and each division line intersects a point of the slope on theside surface of the dot, on which intersection points a light source isdepicted P1, at P2, at P3, and at P4, representing light reflectionpoints. Next, an angle between straight line 32 and dot base 12 throughP2 and P3 is shown, and this is designated as a section inclinationangle. The section inclination angle controls the dispersion of thelight and the efficiency of light reflection, and is an importantrelation.

[0054] By regulating the inclination angle relative to the rangesmentioned above, a distribution of the angles at which light raystraveling through the light guide plate exit the latter can beoptimized, and, at the same time, the axial luminance (i.e., luminanceas viewed in the direction perpendicular to the light-transmissivesurface or a coextensive plane of the backlighting light guide panel)can be increased while suppressing the quantity of light rays exitingobliquely from the light guide plate. That is, when the sectioninclination angle exceeds 35°, the rate of the light that is entirelyreflected at the dot slope is reduced, and the efficiency drops. And,when it is made smaller than 20°, the angle between the light that isreflected at the dot slope and the light-transmissive surface of a lightguide plate does not sufficiently enlarge. Therefore, the efficiency isreduced.

[0055] In accordance with the present invention, it is desirable that inthe section shape in a direction that is perpendicular to the lightsource of a dot, the lower surface of the dot is substantially astraight line. This is because one can decrease the slope with a gradualinclination by making the lower surface of this dot a straight line, andunnecessary light reflection can be decreased. By optimizing the dotshape to form a small concave as mentioned above, in comparison with acircular dot or a square dot, the brightness is improved by 10%, and incomparison with an ink dot, the brightness is improved by 20% or more.

[0056] In accordance with the present invention, light is projectedoutside of the light guide plate 2 by reflection and refraction from thesloped surface of a dot, and light is spread to every nook and corner inthe light guide plate 2 by using a positive reflection of a part of thelight traveling in the light guide plate 2. Therefore, loss at the timeof a reflection and refraction can be reduced by making the surfacecoarseness of the slope surface of a light guide plate small, so that abrightness improvement can be achieved. Because the surface coarsenessof a light guide plate surface becomes small compared to the slopesurface, it is important to repress the surface coarseness of a slopesurface especially.

[0057]FIG. 8 shows the relation between the face coarseness (RA)of theslope surface (slope between P2 and P3) of the dot of a light guideplate and front face brightness. As for RA, 0.4 μm or less is desirable.

[0058] As for the density of a dot, it is desirable that to achieveequalization of the brightness distribution, the nearer to the lightsource 1, the smaller is the dot density to be. Concretely, when thelength in the Y direction 17 of a light guide plate is 191 mm±20 mm, thethickness of the entrance of the light guide plate 2 is 2.3 mm±0.5 mm,the thickness of the part that is farthest from the light source 1 oflight guide plate 2 is 1.3 mm±0.3 mm, and the Y direction distributionof a light guide plate center is as shown in the graph of FIG. 9,[wherein the abscissa shows distance from the light source (L)/length oflight guide plate in the Y direction L₀] or Z=L/L₀, and the ordinateshows the reflection validness area ratio (RV). The reflection validnessarea ratio (RV))=[Dot length×Dot height/sin (Section inclinationangle)×Number of dots per a unit area]÷Unit area. The reflectionvalidness ratio desirably should exist in the range 33 that is shown incross-hatching in FIG. 9, i.e. 0.5<RV<0.95.

[0059] Concretely, the reflection validness area ratio is as follows,determined in the range of 0.05<RV<0.95, K₁Z⁵+K₂Z⁴+K₃Z³+K₄Z²+K₅Z^(1a+K)₆+C1<the reflection validness area ratio ofK₁Z⁵+K₂Z⁴+K₃Z³+K₄Z²+K₅Z¹+K₆+C2 Here,

[0060] K₁=−0.2330335,

[0061] K₂=+0.7497230,

[0062] K₃=−0.6375126,

[0063] K₄=+0.1875481,

[0064] K₅=−0.0011018,

[0065] K₆=+0.0298941,

[0066] C1=−0.004,

[0067] C2=+0.004.

[0068] Uniform brightness can be gained over the entire liquid crystaldisplay surface by setting the reflection validness area ratio accordingto this distribution. At this time, the reflection validness area ratiois calculated by using numeric expression 1. The method of specifying adistribution of the dot density by using the reflection validness arearatio makes it possible to properly carry out a design change in case ofchanging the size of a dot and the shape as compared with a method ofusing a dot density and constant dot rate simply. This is because theslope of a long side of the dot in a direction that is far from thelight source 1, as shown in FIG. 6, the dot base 12 and the slope 31 ofa short dot side almost have the function that takes out light.

[0069] The dots or reflecting slant portions should preferably bedisposed at random. Otherwise, moire phenomenon will make an appearancedue to interference of the dot array with other regular patterns, suchas patterns of liquid crystal cells, the color filter, TFT patterns(thin-film transistor pattern) and/or a black stripe array, because thedots of the present invention are minute.

[0070] Rear illumination equipment in which the light guide plate of thepresent invention and the polarizability film of a reflection type(cholesteric liquid crystal film and ¼ phase board) were combined has alarge brightness improvement effect as compared to rear illuminationequipment formed by the combination of a light guide plate using inkdots, a cholesteric liquid crystal film and ¼ phase sheet, or rearillumination equipment in which the light guide plate that is formedwith grating grooves, a cholesteric liquid crystal film and ¼ phaseboard were combined. In addition, the light guide plate of the presentinvention is easier to manufacture than the light guide plate that isformed with grating grooves, and it is desirable when the light guideplate of the present invention, a cholesteric liquid crystal film and ¼phase board are combined.

[0071] In the following, various embodiments of the present inventionwill be explained with reference to the drawing.

[0072] (Embodiment 1)

[0073]FIG. 1 is the perspective view of the rear illumination equipmentthat is used for the liquid crystal display of the present invention.FIG. 6 shows shape of the dot (small concave) as an example. In thisexample, the length 28 of a dot was made 200 μm, the width 29 of a dotwas made 40 μm, and the depth of a dot was made 8 μm. This rearillumination equipment includes light source 1, light guide plate 2 andreflection sheet 4 as main components, and in light guide plate 2, aplurality of dots are formed in the under surface thereof. And, the dotsare randomly arranged. An example of the dot assignment is shown in FIG.10. Moire can be prevented from occurring by using such a randomassignment. And, in this example, a diffusion sheet 5 and two condensingsheets 6 and 7 are used in addition to the main components, as seen inFIG. 2.

[0074]FIG. 11 shows the dot section shape of light guide plate 2manufactured according to this example. The section inclination angle is28°. FIG. 12 illustrates the average brightness of the sectioninclination angle, according to this example, relative to the lightguide plate. As for the dot section inclination angle, as understoodfrom FIG. 12, 20°-35° is pertinent, and an optimum value is 28°±3°. FIG.13 shows the distribution of the reflection validness area ratio of adot.

[0075] (Embodiment 2)

[0076]FIG. 17 is a structural diagram of a liquid crystal display usinga cholesteric liquid crystal film. We compared and examined eachbrightness improvement effect of Embodiment 2, that uses the light guideplate of the present invention for the light guide plate in FIG. 17,with a comparison example 1 using the light guide plate of an ink dotmethod and comparison example 2 using a light guide plate with gratinggrooves.

[0077]FIG. 18(a) and FIG. 18(b) are graphs showing the brightnessdistribution in the X direction and the Y direction in above embodiment2 in comparison with the brightness distribution in the X direction andthe Y direction for those cases in which the cholesteric liquid crystalfilm and λ/4 sheet, respectively, are removed from the structure ofembodiment 2. Anything in an X direction and Y direction has 20%brightness improvement effects with the present invention.

[0078]FIG. 19(a) and FIG. 19(b) are graphs showing the brightnessdistribution in the X direction and the Y direction in above comparisonexample 1 in comparison with the brightness distribution in the Xdirection and the Y direction for those cases in which the cholestericliquid crystal film and λ/4 board, respectively, are removed from thestructure of comparison example 1. Anything in an X direction and Ydirection has 10% brightness improvement effects.

[0079]FIG. 20(a) and FIG. 20(b) are graphs showing the brightnessdistribution in the X direction and the Y direction in above comparisonexample 2 in comparison with the brightness distribution in the Xdirection and the Y direction for those cases in which the cholestericliquid crystal film and λ/4 sheet, respectively, are removed from thestructure of embodiment 1. Anything in an X direction and Y directionhas 20% brightness improvement effects.

[0080] As for the light guide plate according to the present invention,a brightness improvement effect on the cholesteric liquid crystal filmis higher than in a light guide plate formed of the ink dot method; and,it has a brightness improvement effect that is equal to the light guideplate having grating grooves. In addition, even in a case where acholesteric liquid crystal film is not used, the brightness of theliquid crystal display using a light guide plate according to thepresent invention is 10% or more higher than one formed by an ink dotmethod, and more than equal to one having grating grooves. In addition,the present invention is excellent as compared to the use of gratinggrooves from the point of view of the uniformity of the brightnessdistribution in the face and the ease of metal mold manufacturing.

[0081] Table 1 is a table concerning the liquid crystal display of thepresent invention and the liquid crystal display of the above comparisonexample, in which the brightness change in the presence or absence of acholesteric liquid crystal film and the associated problems have beensummarized. TABLE 1 COMPARISON OF EACH METHOD LIQUID CRYSTAL DISPLAYCOMPARISON COMPARISON EXAMPLE 1 EXAMPLE 2 EMBODIMENT 2 INK DOT METHODGRATING METHOD POLARIZABILITY FILM OF A *1 REFLECTION TYPE THERE THERETHERE NOTHING IS NOTHING IS NOTHING IS FRONT FACE 111 135 100 108 110132 BRIGHTNESS *2 PROBLEM IN CASE NOTHING NOTHING NOTHING NOTHINGNOTHING THE X OF MASS- DIRECTION PRODUCING BRIGHTNESS DISTRIBUTION ISDIFFICULT TO AMEND. IT IS DIFFICULT TO MANUFACTURE A METAL MOLD COST ⊚ ∘∘ Δ x xx INTERPRETATION ∘ ∘ Δ Δ x x

[0082] As can be seen, a light guide plate according to the presentinvention has brightness that is more equal than that of the light guideplate of a conventional ink dot method and a liquid crystal displayusing a cholesteric liquid crystal film, and the present invention ispredominant in the point of production cost. In addition, in case highbrightness is necessary, the liquid crystal display that uses a lightguide plate according to the present invention combined with acholesteric liquid crystal film is excellent. And, even if apolarizability film other than a cholesteric liquid crystal film of areflection type is used, the same effect can be gained.

[0083] Now, a method of manufacturing a light guide plate for rearillumination equipment according to the present invention will beexplained.

[0084] As a method of manufacturing a light guide plate, a metal mold ismanufactured, and the light guide plate is plastically molded using themetal mold. As for the number of the dots that consist of small concaveson the light guide plate, applying the manufacturing method that isdescribed below so that the light guide plate becomes a 12.1 inch plate,500,000-2,000,000 dots are provided in the light guide plate, and thefact that it becomes a huge number is fine.

[0085]FIG. 14 is a process flow diagram that illustrates themanufacturing method. The present invention has the following processes.

[0086] (1) Process that forms photo resistance 35 on substrate 34;

[0087] (2) Process that develops a photo resist layer after arranging amask 36 with the dot pattern above the substrate 34 and irradiatingultraviolet rays 37 from the upper part of mask 36 to form the dotpattern on the substrate 34;

[0088] (3) Process that forms a stamper 39 that consists of platinglayers 38 formed by metal plating on the dot pattern; and

[0089] (4) Process which, by using the stamper 39. light guide plate 2is plastically molded.

[0090] A glass plate, etc. ground to have a mirror surface is used asthe substrate 34 in this process. Before forming photo resist layer 35,an adhesive improvement agent of a silane system can be applied inadvance. As a photo resist material, a liquid-form, a positive film-formtype and a material of a negative type are available. There are a methodof spin coating and a method of roll coating as possible film formationmethods. It is possible to change the depth of a small concave bycontrolling the thickness of the photo resist layer. And, by setting theconditions of exposure, development and annealing, the sectioninclination angle can be controlled. Various masks, such as a chromemask, a film mask and an emulsion mask, can be used for the photo mask36, which are designed to establish the size and the number of dots inadvance. Data, such as the distribution and a drawing by an electronbeam, a laser beam, etc. are also determined in advance. When aconduction film is formed to form stamper 39, before forming platinglayer 38, plating layer that is, 38 of the uneven of plating that is notand is favorable. It is a material of which while as a material of aconduction layer and a plating layer, various metals can be used, but Ni(nickel) is optimum in the point of uniform and mechanical performance.Obtained plating layer 38 can exfoliate physically and easily fromsubstrate 34. It is finished by grinding according to necessity and isused as stamper 39.

[0091] Obtained stamper 39 is fixed to the matrix of, for example, aninjection molding machine with a magnet, a vacuum chuck, etc., and lightguide plate 2 is plastically molded. It is possible to mold light guideplate 2 by ejection molding and compression, a vacuum molding, etc.other than by using the above method of manufacturing the light guideplate 2 by an injection molding machine as shown.

[0092] As a material that constitutes the light guide plate 2, a plastictransparent material is available. There are an acrylic system plastic,a polycarbonate resin, a polyacetal resin, a polyurethane system resinand a plastic material of an ultraviolet hardening type. An acrylicsystem material is a material that is considered excellent from thepoint of view of transparency, price and moldability and is suitable fora present invention.

[0093] Now, the structure of a liquid crystal display will be explained.

[0094] The main unit structure of the liquid crystal display of thepresent invention is shown in FIG. 15. A deflection board, a liquidcrystal cell, a common electrode, a color filter and a polarizationlayer are installed over the light emitting surface of the rearillumination equipment. This structure represents a general example of aliquid crystal display. For example, an especially wide angle ofvisibility is required from the liquid crystal display of a desk toptype display unit or a television monitor of a personal computer. Thediffusion board that makes the angle of visibility expand about whichillumination light is scattered in this case can be arranged in adesired position. And, a prism sheet is provided, and a sheet that hasan optical diffusion effect to spread the angle of visibility, afterirradiating a liquid crystal cell with illumination light with furtherhigh directivity. The optical scattering function is made to hold, andthe angle of visibility can be spread. As an embodiment of the lightsource 1, a cold cathode pipe, a hot cathode pipe, a tungsten lamp, axenon lamp, a metal halide lamp, etc. can be used. The light source of alow temperature system like a normally cold cathode pipe is desirable.

[0095] The liquid crystal device to which the present invention can findgeneral application and is never restricted to any specific application,but can be applied to conventional liquid crystal devices or liquidcrystal device panels known heretofore. As the liquid cell arrays towhich the invention can find application, there may be mentionedgenerally twist mnematic, super-twist mnematic, homogenous, thin filmtransistor or the like type, or a liquid crystal cell array of theactive matrix driving type or a simple matrix driving type.

[0096] As to the reflection polarizability film that is used for thepresent invention, the invention is not especially limited, andwell-known films such as a cholesteric liquid crystal film can be used.Concretely, DBEF (trademark) manufactured by the 3M company, TRANSMAX(trademark), etc. manufactured by the MERCK company, can be used.

[0097] As described, a liquid crystal display having a multifunction andhighly efficient characteristics that are stabilized without generationof uneven brightness, with an improved brightness, is provided by thepresent invention.

What is claimed is:
 1. A liquid crystal display, comprising: a liquidcrystal display panel having a plurality of liquid crystal cells, a backlight that irradiates the liquid crystal display panel with irradiationlight, wherein said back light includes: a light source, and a lightguide plate having one side arranged adjacent said light source andhaving a planar surface with a plurality of small concaves, said planarsurface being almost parallel to a liquid crystal cell face, and whereina plane shape of said small concaves is almost rectangular.
 2. A liquidcrystal display according to claim 1, wherein a lower portion of saidsmall concaves has a length in a direction that is parallel to an axisof illumination of said light source in the range of 80-800 μm, and alength in a direction that is perpendicular to the axis of illuminationof said light source of 60 μm or less.
 3. A liquid crystal displayaccording to claim 1, wherein said small concaves have a depth in therange of 5-20 μm.
 4. A liquid crystal display according to claim 1,wherein side surfaces of said small concaves have an inclination anglein the range of 20°-35°.
 5. A liquid crystal display according to claim1, wherein said small concaves have surfaces with a face coarseness ofRA<0.4 μm.
 6. A liquid crystal display according to claim 1, whereinsaid small concaves are randomly arranged.
 7. A liquid crystal displayaccording to claim 1, further comprising: a polarizer film of areflection type which is arranged on the side of the light guide platelight-transmissive surface of the light guide plate.
 8. A liquid crystaldisplay according to claim 1, further comprising: a cholesteric liquidcrystal film which is arranged on the side of the light guide platelight-transmissive surface of the light guide plate.
 9. A liquid crystaldisplay according to claim 1, further comprising: a cholesteric liquidcrystal film and ¼ phase plate which is arranged on the side of thelight guide plate light-transmissive surface of the light guide plate.10. A liquid crystal display, comprising: a liquid crystal display panelhaving a plurality of liquid crystal cells; a back light that irradiatessaid liquid crystal display panel with irradiation light; a prism sheetwhich is located between said liquid crystal display panel and said backlight; wherein said back light includes: a light source; a light guideplate having one side arranged adjacent said light source and having aplanar surface with a plurality of small concaves, said planar surfacebeing substantially parallel to a liquid crystal cell face; wherein aplane shape of said small concaves is substantially rectangular; andwherein side surfaces of said small concaves have an inclination anglein the range of 20°-35°.
 11. A liquid crystal display according to claim10, wherein the side surface of said small concaves have an inclinationangle in the range of 25°-31°.
 12. A liquid crystal display, comprising:a liquid crystal display panel having a plurality of liquid crystalcells, a back light that irradiates the liquid crystal display panelwith irradiation light, wherein said back light includes: a lightsource, and a light guide plate having one side arranged adjacent saidlight source and having a planar surface with a plurality of smallconcaves, said planar surface being almost parallel to a liquid crystalcell face, wherein a plane shape of said small concaves is almostrectangular, and wherein a lower portion of said small concaves has alength in a direction that is parallel to an axis of illumination ofsaid light source in the range of 80-800 μm, and a length in a directionthat is perpendicular to the axis of illumination of said light sourceof 60 μm or less.
 13. A liquid crystal display according to claim 12,wherein said small concaves have a depth in the range of 5-20 μm.
 14. Aliquid crystal display, comprising: a liquid crystal display panelhaving a plurality of liquid crystal cells, a back light that irradiatesthe liquid crystal display panel with irradiation light, wherein saidback light includes: a light source, and a light guide plate having oneside arranged adjacent said light source and having a planar surfacewith a plurality of small concaves, said planar surface being almostparallel to a liquid crystal cell face, wherein a plane shape of saidsmall concaves is almost rectangular, wherein a lower portion of saidsmall concaves has a length in a direction that is parallel to an axisof illumination of said light source in the range of 80-800 μm, and alength in a direction that is perpendicular to the axis of illuminationof said light source of 60 μm or less, wherein said small concaves havea depth in the range of 5-20 μm, and wherein side surfaces of said smallconcaves have an inclination angle in the range of 20°-35°.